Air-conditioning systems used in electric vehicles cannot perform a heating operation using combustion exhaust heat from engine cooling water etc. because they are not equipped with an engine. Although exhaust heat from a driving motor, a battery, etc. substituting for an engine can be used, the amount of exhaust heat is so small that a heating system using only exhaust heat as a heat source cannot be established. On the other hand, although a heating system using an electric heater is conceivable, this has the problem of significantly decreasing the driving mileage of the vehicle because power consumption for heating is large relative to the battery capacity.
Thus, a heat pump system equipped with an electric compressor has been considered as an air-conditioning system for electric vehicles. However, a reverse-cycle heat pump in which a refrigerant circuit is switched to use a condenser as an evaporator and an evaporator as a condenser during heating has some problems: at a low outside air temperature, the condenser, which is installed outside the vehicle interior, needs a defrosting operation (during heating, it functions as an evaporator), the heating capacity is insufficient, a dehumidifying operation is impossible, windows fog up when switching between cooling and heating, and so on.
Furthermore, with the foregoing reverse-cycle heat pump, in the case of a heating operation under low outside air temperature conditions, the refrigerant evaporation action of the evaporator (which functions as a condenser during cooling) installed outside the vehicle interior changes considerably, which may cause a liquid slugging operation when starting up or an abnormal low pressure operation, exerting an excessive load on the compressor, thus causing damage to the compressor. To prevent the occurrence of such situations, an accumulator or intake-refrigerant pressure detecting means is disposed at the intake side of the compressor; however, this has the problems of complicating the system and increasing the cost.
Therefore, various improvements have been proposed in the related art; for example, the use of not only exhaust heat from a driving motor including an inverter and a battery, or ventilation exhaust heat from a vehicle interior, but also an electric heater or a combustion heater serving as a heat source for heating has been considered because such exhaust heat has insufficient capacity or it takes much time until the exhaust heat can be used (for example, refer to Patent Literatures 1, 2, and 3).
Furthermore, an approach has been proposed in which a positive temperature coefficient heater (PTC heater) is disposed upstream of a refrigerant evaporator or downstream of a second refrigerant condenser in a heating ventilation and air conditioning unit (HVAC unit) using a heat pump system, where it is used as an auxiliary heat source for heating (for example, refer to Patent Literatures 4 and 5).